What Is TEWL?
Transepidermal Water Loss (TEWL) is the passive, continuous evaporation of water from the skin's surface. It's measured in g/m²/h and represents the rate at which water from inside the body diffuses through the epidermis and evaporates into the environment.
Healthy, intact skin has a TEWL of approximately 5–10 g/m²/h. Compromised or severely dry skin can exceed 20–40 g/m²/h — a fourfold increase that leaves skin dehydrated, tight, and reactive.
The Lipid Barrier: Your Anti-TEWL Shield
The stratum corneum's lipid matrix — composed of ceramides (50%), cholesterol (25%), and free fatty acids (15%) — is the primary physical barrier against TEWL. When this "mortar" between corneocyte "bricks" is disrupted, water escapes rapidly.
Causes of lipid barrier disruption include:
- Over-cleansing with surfactant-heavy cleansers (removes native lipids)
- Over-exfoliation with AHAs/BHAs (damages the lipid matrix)
- Cold, dry, windy environments (environmental TEWL)
- Hot showers (lipid extraction)
- Skin conditions: eczema, psoriasis, rosacea (structural barrier defects)
Ingredients That Reduce TEWL
| Ingredient | Mechanism | Reduction Effect |
|---|---|---|
| Petrolatum | Occlusive — forms a physical film | Up to 99% TEWL reduction |
| Dimethicone | Semi-occlusive silicone film | ~60–70% reduction |
| Ceramides | Replenishes lipid matrix | Structural barrier repair |
| Cholesterol | Restores lipid ratio (25% target) | Synergistic with ceramides |
| Squalane | Emollient — softens and seals | Moderate reduction |
| Hyaluronic Acid | Humectant — draws water to surface | Does NOT reduce TEWL; can increase it in dry air |
The humectant trap: Hyaluronic acid attracts water but doesn't seal it in. In low-humidity environments, it can draw moisture from the dermis outward, actually increasing TEWL. Always layer a humectant under an occlusive or emollient.
The Correct Layering Order
- Step 1 — Humectant: Hyaluronic acid, glycerin (draws water to skin)
- Step 2 — Emollient: Squalane, ceramide moisturiser (softens, fills lipid gaps)
- Step 3 — Occlusive: Petrolatum, shea butter (seals everything in)
See our full layering science in the Layering Rules guide. For the glossary definition, visit Glossary: TEWL.